1. Field of the Invention
The present invention relates generally to construction machines of the type including a milling drum supported from a machine frame, including milling machines, surface miners, recyclers, stabilizer machines, and the like.
2. Description of the Prior Art
During the milling of a large road surface area, it is common to use a large milling machine, such as a half lane milling machine, to mill the vast majority of the area of the road surface, leaving only small remainder areas which cannot be engaged by the large milling machine. Those small remainder areas are then later milled by a smaller, more maneuverable milling machine.
When performing a milling operation such as milling the surface of a road, or other ground surface, there are often areas of the road surface which must be avoided by the milling drum in order to prevent damage to the milling drum and/or to the area of the road surface in question. Such areas which it is desirable to avoid with the milling drum may for example include manhole covers, drain gratings, hydrant covers, and in general any area which would be damaged by the milling drum or which poses a threat of damage to the milling drum, or which for any other reason is not to be milled.
Thus, for example, in the process of milling a road, when the milling drum approaches the location of a manhole cover, it is desirable to mill to a location near to the manhole cover, then to raise the drum and pass it over the manhole cover then lower the drum back into milling contact with the road surface.
Current practice for dealing with such obstacles is generally as follows:                1. A milling machine operator or driver and a ground operator work as a team. The ground operator walks along beside the milling machine, and will visually look for the obstacles such as manhole covers in the path of the milling machine. The ground operator will mark the location of the obstacles by spraying lines on the ground with a highly visible paint. The ground operator and an assistant operator will typically pull a string perpendicular to the path of the milling machine at the beginning edge and ending edge of the obstacle, and will paint straight lines parallel to the string laterally out to the side of the machine path so that the lines are visible when the obstacle disappears beneath the milling machine.        2. Then the ground observer must determine where the location of the forward cutting edge of the milling drum is within the drum housing. It will be appreciated that as the milling depth increases, the effective cutting length of the milling drum in the direction of travel increases and thus the location of the intersection of the milling drum with the road surface moves forward relative to the side plate of the milling drum housing. Typical milling machines have a pictorial chart displayed on each sideplate of the milling drum housing that indicates where the forward cutting edge of the drum is located for various milling depths. This is indicated by reference to various bolts or other features near the lower edge of the sideplate. With this chart the ground observer identifies where the forward cutting edge of the milling drum is located within the drum housing.        3. The ground operator will warn the milling machine driver when an obstacle is near. Then the ground operator will communicate to the milling machine driver when advancement of the milling machine should stop. This communication is typically performed by hand signals. The decision by the ground operator as to when to stop advancement of the milling machine is a subjective judgment, and the ground operator must err on the side of avoiding contact of the milling drum with the obstacle. Because of the uncertainties in this existing procedure, the ground operator will typically instruct the milling machine operator to stop milling earlier than is actually necessary to avoid contact with the obstacle. This results in an increased remainder area which must later be milled at higher cost by a smaller, more maneuverable milling machine.        4. Upon receiving the signal to stop, the milling machine driver will stop advancement of the milling machine and raise the milling drum. The milling machine driver then again advances the milling machine with the drum raised until the ground operator again signals the milling machine driver to stop and to lower the milling drum back into milling engagement with the ground. This second decision by the ground operator is again a subjective one, and care must be used to avoid lowering the milling drum too early and hitting the back edge of the obstacle.        
There are several difficulties encountered in the process just described. One difficulty is the inability of the ground observer to actually see either the obstacle or the milling drum, and thus the requirement for a subjective judgment to be made as to when to raise and when to lower the milling drum. Another difficulty is the communication between the milling machine driver and the ground operator due to noise and other adverse conditions at the job site. Both of these difficulties are increased when the milling job is being done at night, which makes it even harder for the ground observer to locate obstacles, and for the ground observer and the milling machine driver to communicate with hand signals.
As a result of these difficulties there is a lack of consistency in performance by various operator teams in milling close to obstacles. The end result is greatly dependent upon the skill and experience of the operator team. Some teams may be able to consistently mill within a couple of inches of obstacles without striking the obstacle. Other teams may leave as much as ten inches or more of un-milled material on either side of an obstacle and/or may repeatedly damage equipment by striking obstacles.
Thus there is a need for an improved system for avoiding obstacles or areas which are not to be milled during the operation of a large milling machine.